WO1995021945A1

WO1995021945A1 - A method of removing halogens from a zinc containing material

Info

Publication number: WO1995021945A1
Application number: PCT/SE1995/000154
Authority: WO
Inventors: Bror Erik SKÖLD; Guozhu Ye
Original assignee: Odda Recycling As
Priority date: 1994-02-15
Filing date: 1995-02-15
Publication date: 1995-08-17
Also published as: SE9400533D0; SE9400533L; AU1828995A

Abstract

In the recycling of steel, the galvanized and painted steel increases its proportion all the time and the zinc is collected mostly as dry or wet dust. When this dust is worked up, the halogens from the paint causes a problem. By evaporating zinc and halides and oxidizing the zinc and selectivey sulphatizing the halides, the halogens can be separated from the zinc oxide. Freedom of halogens increases considerably the value of the zinc oxide.

Description

A method of removing halogens from a zinc containing material

This invention relates to a method of removing halogens from a zinc containing material by a sulphatizing process.

In the steel industry large amounts of dry and wet zinc containing dust is collected. Dust from electric arc furnaces contains usually 25 - 35 % Fe and 25 - 35 % Zn. The zinc comes from the melting of galvanised steel. The dust contains usually 1-10 % Cl and 0.2-1.5 % F that come from paint. When the dust is worked up, most of the halogen content will be in the resulting ZnO-product which reduces considerably the value of the ZnO-product. The halogens are usually in an insoluble form and cannot be removed by leaching.

It is an object of the invention to make it possible to remove in a simple and economic way halogens from a Zn containing material, for example from dry or wet dust collected in the recycling steel industry.

According to the invention, in principle, zinc is evaporated from the zinc containing material and oxidized while evaporated halides are sulphatized at such high a temperature that zinc sulphate is unstable. Then, the halogens are separated from the zinc. The invention has been given the characteristics stated in the claims.

The zinc can be collected as solid matter (mainly zinc oxide) and the halogens may pass the collection as gaseous hydrogen halides and they can be collected from the fumes that passes the collection of solid matters. Alternatively, the halogens may be collected as solid matters (mainly alkali halides or alkalene earth metal halides) and possibly also as condensed hydrogen halides together with the zinc oxide, and the halides can be separated from the zinc oxide by a leaching process. The leaching can be carried out for example with pure water or with a solution of washing soda in water. It is possible to leach since the halides generated from the sulphates are soluble. The invention will be described in more detail with reference to the Figure which show an example of a plant which can be used to carry out the invention.

The Figure shows an electric direct arc furnace 9 of the D C-type (direct current). It has an electrode 10 with a central through channel 11. From bins 12, 13 and 14 zinc containing dust, reduction agent, and slag forming agent, respectively, can be supplied to the channel 1 1 by a conveyor 15. The slag formed in the furnace and liquid iron at the bottom of the furnace 9 can be tapped through a tap hole 15. The exhaust pipe 16 has an after-burner 17, with a supply pipe 18 from a non illustrated mixing chamber, and downstream the after-burner 17 there is a cooler in the form of a heat exchanger or a conditioning tower 19, a collector in the form of an electric filter 20, and a scrubber 21. From the scrubber 21, the exhaust gases are conveyed through a fan 22 to a chimney 23.

In operation, dust, reduction agent (e.g. coal dust) and sand are supplied to the furnace 9 via the electrode 10. The hot plasma in the furnace 9 reduces and melts iron oxide to liquid hot iron or to FeO in the slag. Zn and other volatile metals such as Pb evaporates. The exhaust gas comprises these fumes together with CO/CO₂. A mixture compising oxygen or air, water vapour, and a sulphatizing agent such as SO₂ or SO₃ or H₂SO , is supplied through the pipe 18. The temperature in the after¬ burner should be so high that the zinc sulphate will be unstable and the sulphatizing process will therefore be selective. The temperature of combustions should preferable be above about 700°C and preferably 800 - 1000°C. Zn and other evaporated metals will oxidize and halides will react as follows:

2NaCl + SO₂ (g) + l/2O₂ (g) + H₂O (g) = Na₂SO₄ + 2HC1 (g) 2KC1 + SO₂ (g) + I/2O2 (g) + H₂O (g) = K₂SO₄ + 2HC1 (g) CaF₂ + SO₂ (g) + I/2O2 (g) + H₂O (g) = CaSO₄ + 2HF (g)

The thermodynamic forces for transforming the halides into sulphates are great. NaCl, KC1 and CaF₂ are the most stable halides that will normally be present in process fumes or ZnO-product. Tests have proved that these halides are transformed into sulphates and thus, all other halides will also be transformed into sulphates.

Collection of the solid matters, that is, the sulphates and the metal oxides as dust, is difficult at such high a temperature as about 800°C. Therefore, the fumes are cooled in the conditioning tower 19 by direct contact with water. The temperature can be about 400°C or lower when an electric filter is used. The amount of sulphatizing agent should be somewhat overstochiometric with respect to the halogens, but the amount should not be too high since zinc sulphate will then be formed during the cooling of the fumes. The amount of zinc sulphate in the collected zinc oxide may in practice be a few percent, it should preferably not exceed 10 % or even 5 %. The zinc is thus collected mainly as ZnO dust.

The halogens can either pass the collection of solid matter as hydrogen halides in gaseous state or they can regenerate as salts and be collected as solid matters together with the metal oxides. A part of the halides may pass the collection as hydrogen halides and a part of the halides may be collected as salts and the relative proportions can be controlled by the control of factors such as temperature.

The hydrogen halides are collected and neutralized in a conventional way in the scrubber 21 before the fumes are conveyed to the chimney 23.

As an alternative to the conditioning tower 19, the filter 20 and the scrubber 21, a single scrubber can be used in which a sludge is collected which is leached and dewatered. The resulting dewatered filter cake comprises the halogen free zinc oxide. In this scrubber, the temperature is thus lowered directly from the reaction temperature of for exemple about 800°C to less than 100°C.

In an upgrading process without sulphatization, the halides collected together with the metal oxides in the filter 20 would not be soluble. Surprisingly, with the sulphatizing process, regenerated halides are soluble and by leaching with water or a solution of alkali carbonate in water, preferably washing soda Na₂ CO₃, they can be removed from the collected oxides and sulphates. The metal oxide dust can then be treated in conventional plants for producing zinc. The sulphates will not be harmful in such a process since they will be removed in a process step that is already present in the conventional plants.

Claims

1 A method of removing halogens from a zinc containing material by a sulphatizing process, characterized by the steps of evaporating zinc, oxidizing the evaporated zinc and selectively sulphatizing evaporated halides at such high a temperature that zinc sulphate is unstable, and separating the halogens from the zinc.

2 A method according to claim 1 , characterized in that the separation is carried out at such high a temperature that the zinc is collected as solid matter, mainly as oxide, while the halogens pass the collection as hydrogen halides in gaseous state, and the hydrogen halides are separately collected from the fumes that passes the collection of solid matters.

3 A method according to claim 1, characterized in that the temperature of the fumes is lowered and the oxidized zinc and the halides are collected together from the fumes and the halogens are leached from the collected matter.

4 A method according to claim 1, characterized in that the temperature of the fumes is lowered and solid matters in the fumes are collected whereafter halogens are leached from the collected matter leaving zinc oxide purified from halogens and halogens are collected from the fumes that passes the collection of solid matters.

5 A method according to claim 3 or 4, characterized in that water with alkali carbonate is used for the leaching.

6 A method according to claim 3, 4 or 5 characterized in that said lowering of the temperature of the fumes is carried out by water in direct contact with the fumes. 7 A method according to any one of claims 2-6, characterized in that an electric filter is used to collect solid matters from the fumes.

8 A method according to claim 3, characterized in that the fumes from the oxidizing and sulphatizing process are treated in a wet scrubber that reduces the temperature of the fumes to less than 100°C and removes the zinc and the halogens from the fumes, and the halogens are washed out from the zinc which is present mainly as zinc oxide.

9 A method according to any one of the preceding claims, characterized in that the sulphatization is carried out at a temperature of above 700°C.

10 A method according to claim 9, characterized in that the sulphatization is carried out at a temperature of 800 - 1000°C.